With the development of various information instruments such as a mobile phone and a personal digital assistance (PDA), many kinds of electronic components such as a resistor, a condenser, and an integrated circuit have been, in a trend of utilizing a high-density mounting technology, mounted more frequently on a printed wiring board (which is also referred to as printed circuit board, rigid substrate, printed substrate, or printed wiring substrate) at a density higher than ever before. The printed wiring board is a sheet-shaped composite material formed by combining a resin, a glass fiber, a modifier, and the like at an appropriate ratio, and has a form in which a through-hole or the like for mounting various kinds of electronic components is provided. The printed wiring board is sometimes expressed by using an alias such as a module, a board, a unit, or a package depending on the functions and applications.
In the production of glass fiber that is used for the printed wiring board, there has been conventionally used a glass composition called E glass, which has an alkali-free glass composition. The E glass is a material that is excellent in electrical insulation, excellent in spinnability necessary for producing a glass fiber from glass in a melted state, and excellent in workability such as cutting workability, and hence the E glass is a glass material for a glass fiber which has been actually used frequently and is most well known. The E glass is a glass material which includes, in terms of oxides by mass %, for example, 52 to 56% of SiO2, 12 to 16% of Al2O2, 5 to 10% of B2O3, 16 to 25% of CaO, 0 to 5% of MgO, 0 to 2% of an alkali metal oxide (R2O), 0.05 to 0.4% of Fe2O3, and 0 to 1.0% of F2.
On the other hand, for use in a printed wiring board, a high-frequency wave has been required to be applied in recent years in order to realize a high-speed electronic circuit. In this case, it is electrical characteristics of the printed wiring board that are emphasized as important. A transmission rate is in inverse proportion to the square root of a dielectric constant, and hence a low dielectric constant is necessary for improving the transmission rate. Further, a small dielectric loss is required, and a small dielectric dissipation factor is necessary for attaining that purpose. Note that the dielectric constant (∈) in the present invention accurately refers to a dimensionless number that means a specific dielectric constant, which is the ratio of the dielectric constant of a medium to the dielectric constant of a vacuum, but the dielectric constant (∈) is used in accordance with the conventional way. In general, when alternating current is applied to glass, the glass absorbs energy with respect to the alternating current by absorbing the energy as heat. Dielectric loss energy that is absorbed is in proportion to a dielectric constant and dielectric dissipation factor which are determined depending on the components and structure of glass, and is represented by W=kfv2×∈ tan δ. Here, W represents the dielectric loss energy, k represents a constant, f represents a frequency, v2 represents a potential gradient, ∈ represents the dielectric constant, and tan δ represents the dielectric dissipation factor. From this equation, it is found that, as the dielectric constant and the dielectric dissipation factor are larger, or as the frequency is higher, the dielectric loss becomes larger. Thus, in order to make the dielectric loss smaller, the dielectric constant and the dielectric dissipation factor are required to be smaller.
Thus, a material for a glass fiber having a low dielectric constant and a low dielectric dissipation factor has been demanded in order to produce a glass fiber used for a printed wiring board. Patent Document 1 discloses a glass material called D glass, which has been developed to realize a dielectric constant and a dielectric dissipation factor lower than a dielectric constant of 6.7 and a dielectric dissipation factor of 12×10−4 of the E glass at a frequency of 1 MHz at room temperature. The D glass is a glass material which includes, in terms of oxides by mass %, for example, 74.5% of SiO2, 0.3% of Al2O3, 21.7% of B2O3, 0.5% of CaO, 0.5% of Li2O, 1.0% of Na2O, and 1.5% of K2O. This glass has a dielectric constant of about 4.3 and a dielectric dissipation factor of about 10 to 20×10−4 at 1 MHz.
However, it was pointed out that the D glass involved various problems in the production processes of a printed wiring board and a glass fiber and the like, though the D glass had excellent electrical performance. For example, the D glass is inferior to the E glass in meltability and frequently has problems such as broken yarns during spinning, and hence the production of a glass fiber is not easy. In addition, because the D glass has a fragile structure, the D glass is inferior in weaving performance in a weaving process that takes place to make a fabric form to be used for obtaining a printed wiring board, leading to reduction in the yield rate of products. Besides, a printed wiring board formed by using the D glass also involved a problem in that, in a drilling process for forming a via hole, which was a through-hole drilled for attaining interlayer conduction without inserting an electronic component lead, the wear of a drill became larger, and hence the accuracy of the position of the via hole lowered.
In order to solve such problems as described above, many inventions have hitherto been made. For example, Patent Document discloses a glass fiber having a low dielectric constant characterized by having, by mass %, a composition of glass of 50 to 60% of SiO2, 10 to 18% of Al2O3, 18 to 25% of B2O3, 0 to 10% CaO, 1 to 10% of MgO, 0 to 1.0% of Li2O+Na2O+K2O, and 0.1 to 1% of Fe2O3.
Patent Document 3 discloses a glass fiber having a low dielectric constant characterized by having, by mass %, a composition of 50 to 60% of SiO2, 10 to 20% of Al2O3, 20 to 30% of B2O3, 0 to 5% of CaO, 0 to 4% of MgO, 0 to 0.5% of Li2O+Na2O+K2O, and 0.5 to 5% of TiO2.
In addition, Patent Document 4 discloses a glass having a low dielectric constant and a low dielectric dissipation factor characterized by having, by mass %, a composition of 48 to 80% of SiO2, 0 to 18% of Al2O3, 11 to 35% of B2O3, 0 to 10% of MgO, 0 to 10% of CaO, 0 to 7% of Li2O+Na2O+K2O, and less than 3% of TiO2, having an H2O content of <800 ppm, and having a dielectric constant of 5.0 or less and a dielectric dissipation factor of 7×10−4 or less at 1 MHz.